1. Field of the Invention
This invention relates to a process for producing cadmium sulfide used as a photoconductive material for electrophotography, and more particularly, to a process for producing the cadmium sulfide doped with a donor impurity of Group IIIa or Group IIIb of the Periodic Table without using a flux.
2. Description of the Prior Art
It is well known that cadmium sulfide shows photoconductivity and is used as a photo-cell or a photosensitive material for electrophotography. However, when cadmium sulfide is in a form of a complete crystal structure, the photoconductivity is low and it is known that the photoconductivity is increased by crystal imperfection of cadmium sulfide itself.
This crystal imperfection of cadmium sulfide is caused by impurity contamination and the resulting photoconductive characteristics depend on the mixing state of the impurity. There are two types of impurities, that is, a donor type and an acceptor type, and examples of the former type impurity are elements of Group III and Group VII and those of the latter type impurity are elements of Group I of the Periodic Table. These impurities are usually introduced into cadmium sulfide by firing.
Cadmium sulfide used for a photo-cell and the process for production thereof are fundamentally different from that used for a photosensitive material for electrophotography and its production process. In the former case, the cadmium sulfide is sintered at an elevated temperature upon introducing the impurity to form coarse crystals since it is necessary for a photo-cell to receive a large electric current.
On the contrary, in the latter case, the particle size of the cadmium sulfide itself directly decides the resolution of electrostatic latent images so that such high temperature firing as in the case of photo-cell can not be employed. Therefore, it is necessary to dope with an impurity suppressing the crystal growth, but the doping can not be effected by firing at low temperatures so that a flux such as CdCl.sub.2 is used. The firing together with a flux at a low temperature is, indeed, effective to suppress the formation of coarse crystals to some extent, but the crystals still grow resulting in crystal size of more than several microns. Consequently, even when a flux is employed, the firing temperature is kept as low as possible, the amount of flux is decreased, a crystal growth inhibiting agent is employed, the diffusion period is shortened and raw CdS of a small particle size is employed.
In view of the foregoing, there have been highly desired processes for producing an activated cadmium sulfide free from coarse crystals without using any flux. The present inventors proposed such process as U.S. application Ser. No, 476,093 filed June 3, 1974.
According to the above mentioned process, a halogen compound such as CdCl.sub.2 is used as a starting material and the chlorine itself works as a donor impurity so that a flux is not necessary. Therefore, this process is a very effective one. However, the amount of chlorine acting as a donor impurity can not be maintained at a constant amount upon washing. This seems to be due to the fact that the chlorine diffused into the CdS is washed away with the washing liquor. Therefore, other treating means are necessary to retain a certain amount of a donor impurity in CdS in a diffused form. As previously mentioned, it is known that there is used an element of Group III or Group VII of the Periodic Table, but such element is relatively easily incorporated into only coarse particles such as particles used for a photo-cell while it is hardly incorporated into fine particles such as particles used for a photosensitive material of electrophotography. In other words, as mentioned previously, the coarse particles are sintered at high temperatures, and the high temperature firing enables a donor impurity to diffuse simply into CdS while the diffusion temperature and amount of flux are restricted in case of fine particles so that the diffusion of the donor impurity is very difficult.